In typical fuel cells, particularly polymer electrolyte, proton exchange membrane (PEM) fuel cells, the interaction between hydrogen fuel and an oxidant (such as air) produces heat, electricity and water. As is known, if the water is not removed from the cathode, it will block the oxidant gas from reaching the cathode catalyst, thereby degrading performance of the fuel cell system.
It is also known that inadequate humidity on the anode side of the PEM will cause ionomer degradation, reducing the useful life of the fuel cell.
Water management has thus far been most successful utilizing porous reactant flow field plates which, when the individual fuel cells are pressed together in a stack, include water transfer channels. The product water tends to flow through the porous plate into the water channels, the water being circulated externally, cooled, and sufficient water returned to the fuel cells. On the anode side, water flowing through the water channels migrates through the porous reactant gas flow field plate toward the anodes to keep the membrane from drying out. A system of this type is disclosed in U.S. Pat. Nos. 5,700,595 and 5,503,944.
Water management which involves circulating water necessarily requires a water pump as well as being subjected to operational problems in freezing temperatures. The net efficiency of a fuel cell power plant is reduced by electricity used to circulate the water, which is referred to typically as parasitic power.
In copending U.S. Pat. No. 6,794,077 B2, issued Sep. 21, 2004, there is disclosed a fuel cell system which does not circulate water externally of the fuel cell. In that case, the water flow channels are dead ended near the air inlet and drain directly into the air exit manifold.
In copending U.S. Pat. No. 6,916,571 B2, issued Jul. 12, 2005, water is circulated by means of convection, from gas bubbles that leak into the water flow fields and from the temperature differential between water within the stack and outside of the stack. In another embodiment in said application, water is not circulated but is allowed to bubble up from the bottom to the top of the cells and then out to ambient atmosphere. In each of these cases, with the water being at atmospheric pressure, the reactant gases must be pressurized in order to provide the pressure differential necessary for water to flow toward the water flow channels, as described in the aforementioned '944 patent. Thus, elimination of a water circulation pump does not necessarily reduce the parasitic power in such a case.